Bill,

I need to see a close up and a further out picture for orientation.  

I also have Standard NACA ducts, although I slightly altered the outline to
meet specs and squared up the ramp to sidewall and sidewall to fuselage
joints.  I have 1/8 inch mesh hardware cloth at the plenum entry to keep out
wasps, eyeballs on the plenum itself to cool my feet, eyeballs on the panel
for face air (fed by 1 in tube), and the passenger plenum also has a 1/2 inch
tube tapped in for radio stack cooling air.  I get air coming in weakly with
just the prop running and plenty during low speed flight right up to the AOA
warning - Angle, angle push!

I can't imagine one could get air sucked out unless the cockpit is
pressurized or there is no way for incoming air ro exit (my friend's E-Racer
pusher had this problem).  Generally, air should be able to get behind the
panel, down the elevator tunnel and out the elevator opening in the vertical
stabilizer

Good Luck!

Scott Krueger